*2.1.1. Improper formulation of vaccine*

The vaccines are manufactured in a processing plant where the titer of antigen of specific virus or bacteria may not be maintained properly; as a result, the inoculums may not initiate protective immune response in birds. The titer of antigen in the vial of vaccine may be low which results in low immunity level in birds. The dose–response relationship among the virus content, serological response and clinical protection has been reported [10]. Virus concentration has a significant effect on immunogenicity of vaccines [11]. The inadequate procedure of formulation of vaccine and lack of standard procedures of vaccine formulation result in the production of nonpotent vaccine.
